Conventional gelcoat compositions, typically formulated from polyester, acrylate, and urethane type resins and combinations thereof are useful as the exterior paint layer for boats and bathroom fixtures such as shower stalls, bath tub enclosures and the like. A gel coat is a pigmented, filled, and prepromoted resin (usually polyester) which is sprayed with an initiator onto molds from a high pressure spray gun to a film thickness of up to 0.75 mm. The film cures prior to reinforcement with glass fibers and laminating resins. The gel coat should exhibit low viscosity at high shear, should resist sagging, and have a gel time of about 8-12 minutes. For marine and other applications, the products require hydrolytic stability and good weatherability.
Use of unsaturated polyesters in admixture with unsaturated aromatic monomers such as styrene used for the production of crosslinked polymers intiated by peroxide is well known. The unsaturated polyesters are prepared from the condensation of unsaturated acids or acid anhydrides with diols and polyols. The most common unsaturated acid used is either maleic anhydride or fumaric acid. Because ester linkages formed from these materials exhibit poor hydrolysis resistance, the overall film performance of a coating film based on these polymers is somewhat poor. In addition, an aromatic diacid, such as phthalic and isophthalic acid, is generally present and the molecular weight of the polymer is maintained above a number average molecular weight of 2000 to achieve the hydrolysis resistance of the film. However, the presence of the aromatic nuclei in the polymer backbone and the necessity to add high levels of unsaturated aromatic monomers to obtain sprayable viscosity lead to poor exterior durability in the coated product.
A coassigned application Ser. No. 728,299 filed concurrently has relevancy to the instant case in that it pertains to the same end use. U.S. Pat. No. 4,304,883 (1981) (Fujii, Tanaka et al) teaches unsaturated polyester resins for coating metal substrate comprising a vinyl monomer and a dicyclopentadiene modified alkyd. U.S. Pat. No. 4,320,221 teaches the reaction of ethylenically unsaturated monofunctional isocyanate and a difunctional polyol as an anerobic adhesive. Other coatings incorporate conventional isophthalic and phthalic based urethane polyesters having various pendant unsaturation.
Urethanes have been incorporated into useful coatings via various techniques. U.S. Pat. No. 3,928,299 (1975) teaches vinyl solution polymers which contain urethane groups and are crosslinkable by vinyl or vinylidene polymerization. The urethane groups are introduced by reacting hydroxy groups pendant from the acrylate backbone with isocyanoalkyl acrylates or methacrylates. Urethane modified epoxy ester resins are taught in U.S. Pat. No. 3,478,126.
Unruh and Smith, U.S. Pat. No. 2,887,469 (1959), teach urethane derivatives of hydroxy containing polymers such as ethyl cellulose, cellulose acetate using monohaloalkyl isocyanate and quaternary urethanes formed with tertiary amines. U.S. Pat. No. 2,768,154 teaches polymeric carbalkoxy urethane via the reaction of carbalkoxy isocyanate (O.dbd.C.dbd.N--R--COOR.sup.1) with hydroxy functional polymers. U.S. Pat. Nos. 4,446,175 and 4,436,885 (1984) teach isocyanate-functional acrylic solvent copolymers of 2-isocyanotoethyl(meth)acrylate with other acrylate monomers in the presence of a mercaptan chain-transfer agent.
U.S. Pat. No. 4,320,221 (1982) teaches the reaction of ethylenically unsaturated monofunctional isocyanate and a difunctional polyol as an anerobic adhesive.
U.S. Pat. No. 4,287,323 (1981) teaches addition polymerizable polyethers having pendant ethylenicaly unsaturated urethane groups as useful flooring materials. Such urethane polyethers are prepared by reacting a polyether polyol (hydroxy terminated polypropylene glycol) with an isocyanatoalkyl(meth)acrylate.
U.S. Pat. No. 4,273,690 (1981) teaches a graft copolymer binder for coatings having an acrylic backbone and alkyd resin chains attached to the backbone by an isocyanate group reactive with the hydroxyl or carboxylic group of the alkyd resin.
Others have attempted to reduce gel coat problems such as blistering or extended water exposure by increasing the film thickness of the gel coat (See Denoms et al, Annual Conference, Reinforced Plastics/Composites Institute, The Society of the Plastics Industry, Inc., Feb. 7-11, 1983).
Vinyl esters based on aromatic polyepoxide resin reacted with unsaturated monocarboxylic acid and other moieties are exemplified in U.S. Pat. Nos. 3,367,992; 3,066,112; and 3,179,623. The presence of the aromatic nuclei and the necessity of adding high levels of unsaturated aromatic monomers to obtain a sprayable viscosity leads to unacceptable exterior durability while those based on aliphatic polyepoxides show poor hydrolysis resistance.
The above deficiencies have been largely overcome by the gel coats of the present invention based on specific hydroxy functional urethane extended polyester oligomers. The improved coatings provide gel coats with low viscosity and excellent hydrolytic and weathering stability.